1. Field of the Invention
This invention relates in general to secondary radar devices and in particular to means for distinguishing between desired and undesired response signals.
2. Description of the Prior Art
Prior art systems are known in which secondary radar devices are confused by signals from other secondary radar devices which give rise to the so-called fruit interference. For example, the book entitled Sekundar-Radar, in Chapter 1, point 7, pages 54-61, by Honold, discloses the cause of and method of eliminating such interference.
The publication "Journal of the British IRE" 16, (1956), pages 355-382 states at page 373 that the pulse repetition rate in secondary radar devices may be obtained from the pulse repetition rate of an associated primary radar device.
The publication "Electronic Engineering" 33, (1961), pages 414-420 states at page 420 that fruit interference can occur when the interrogation pulse repetition rates of neighboring secondary radar devices are the same. This article states that a possible remedy to this problem is to change the interrogation pulse repetition rate of one of the secondary radar devices. There is a disadvantage to this, however, in that when a large area is densely covered by numerous secondary radar interrogation devices there is the danger that the interference will occur in respect to a different pair of secondary radar devices. A further disadvantage consists in the fact that the possibilities of altering the interrogation pulse repetition rate are limited.